Antisense repression of proto-oncogene c-Cbl enhances activation of the JAK-STAT pathway but not the ras pathway in epidermal growth factor receptor signaling

HB-EGF activates EGFR to induce reactive neural stem cells in the mouse hippocampus after seizures

Hippocampal seizures mimicking mesial temporal lobe epilepsy cause a profound disruption of the adult neurogenic niche in mice. Seizures provoke neural stem cells to switch to a reactive phenotype (reactive neural stem cells, React-NSCs) characterized by multibranched hypertrophic morphology, massive activation to enter mitosis, symmetric division, and final differentiation into reactive astrocytes. As a result, neurogenesis is chronically impaired. Here, using a mouse model of mesial temporal lobe epilepsy, we show that the epidermal growth factor receptor (EGFR) signaling pathway is key for the induction of React-NSCs and that its inhibition exerts a beneficial effect on the neurogenic niche. We show that during the initial days after the induction of seizures by a single intrahippocampal injection of kainic acid, a strong release of zinc and heparin-binding epidermal growth factor, both activators of the EGFR signaling pathway in neural stem cells, is produced. Administration of the EGFR inhibitor gefitinib, a chemotherapeutic in clinical phase IV, prevents the induction of React-NSCs and preserves neurogenesis.

miR-22-3p enhances the intrinsic regenerative abilities of primary sensory neurons via the CBL/p-EGFR/p-STAT3/GAP43/p-GAP43 axis

Spinal cord injury (SCI) is a devastating disease. Strategies that enhance the intrinsic regenerative ability are very important for the recovery of SCI to radically prevent the occurrence of sensory disorders. Epidermal growth factor (EGF) showed a limited effect on the growth of primary sensory neuron neurites due to the degradation of phosphorylated-epidermal growth factor receptor (p-EGFR) in a manner dependent on Casitas B-lineage lymphoma (CBL) (an E3 ubiquitin-protein ligase). MiR-22-3p predicted from four databases could target CBL to inhibit the expression of CBL, increase p-EGFR levels and neurites length via STAT3/GAP43 pathway rather than Erk1/2 axis. EGF, EGFR, and miR-22-3p were downregulated sharply after injury. In vivo miR-22-3p Agomir application could regulate CBL/p-EGFR/p-STAT3/GAP43/p-GAP43 axis, and restore spinal cord sensory conductive function. This study clarified the mechanism of the limited promotion effect of EGF on adult primary sensory neuron neurite and targeting miR-22-3p could be a novel strategy to treat sensory dysfunction after SCI.

"Do We Know Jack" About JAK? A Closer Look at JAK/STAT Signaling Pathway

Janus tyrosine kinase (JAK) family of proteins have been identified as crucial proteins in signal transduction initiated by a wide range of membrane receptors. Among the proteins in this family JAK2 has been associated with important downstream proteins, including signal transducers and activators of transcription (STATs), which in turn regulate the expression of a variety of proteins involved in induction or prevention of apoptosis. Therefore, the JAK/STAT signaling axis plays a major role in the proliferation and survival of different cancer cells, and may even be involved in resistance mechanisms against molecularly targeted drugs. Despite extensive research focused on the protein structure and mechanisms of activation of JAKs, and signal transduction through these proteins, their importance in cancer initiation and progression seem to be underestimated. This manuscript is an attempt to highlight the role of JAK proteins in cancer biology, the most recent developments in targeting JAKs, and the central role they play in intracellular cross-talks with other signaling cascades.

Potential role of signal transducer and activator of transcription (STAT)3 signaling pathway in inflammation, survival, proliferation and invasion of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most lethal malignancies, and is also the fourth most common cancer worldwide with around 700,000 new cases each year. Currently, first line chemotherapeutic drugs used for HCC include fluorouracil, cisplatin, doxorubicin, paclitaxel and mitomycin, but most of these are non-selective cytotoxic molecules with significant side effects. Sorafenib is the only approved targeted therapy by the U.S. Food and Drug Administration for HCC treatment, but patients suffer from various kinds of adverse effects, including hypertension. The signal-transducer-and-activator-of-transcription 3 (STAT3) protein, one of the members of STATs transcription factor family, has been implicated in signal transduction by different cytokines, growth factors and oncogenes. In normal cells, STAT3 activation is tightly controlled to prevent dysregulated gene transcription, whereas constitutively activated STAT3 plays an important role in tumorigenesis through the upregulation of genes involved in anti-apoptosis, proliferation and angiogenesis. Thus, pharmacologically safe and effective agents that can block STAT3 activation have the potential both for the prevention and treatment of HCC. In the present review, we discuss the possible role of STAT3 signaling cascade and its interacting partners in the initiation of HCC and also analyze the role of various STAT3 regulated genes in HCC progression, inflammation, survival, invasion and angiogenesis.

The Casitas B lineage lymphoma (Cbl) mutant G306E enhances osteogenic differentiation in human mesenchymal stromal cells in part by decreased Cbl-mediated platelet-derived growth factor receptor alpha and fibroblast growth factor receptor 2 ubiquitination

Human bone marrow-derived mesenchymal stromal cells (hMSCs) have the capacity to differentiate into several cell types including osteoblasts and are therefore an important cell source for bone tissue regeneration. A crucial issue is to identify mechanisms that trigger hMSC osteoblast differentiation to promote osteogenic potential. Casitas B lineage lymphoma (Cbl) is an E3 ubiquitin ligase that ubiquitinates and targets several molecules for degradation. We hypothesized that attenuation of Cbl-mediated degradation of receptor tyrosine kinases (RTKs) may promote osteogenic differentiation in hMSCs. We show here that specific inhibition of Cbl interaction with RTKs using a Cbl mutant (G306E) promotes expression of osteoblast markers (Runx2, alkaline phosphatase, type 1 collagen, osteocalcin) and increases osteogenic differentiation in clonal bone marrow-derived hMSCs and primary hMSCs. Analysis of molecular mechanisms revealed that the Cbl mutant increased PDGF receptor α and FGF receptor 2 but not EGF receptor expression in hMSCs, resulting in increased ERK1/2 and PI3K signaling. Pharmacological inhibition of FGFR or PDGFR abrogated in vitro osteogenesis induced by the Cbl mutant. The data reveal that specific inhibition of Cbl interaction with RTKs promotes the osteogenic differentiation program in hMSCs in part by decreased Cbl-mediated PDGFRα and FGFR2 ubiquitination, providing a novel mechanistic approach targeting Cbl to promote the osteogenic capacity of hMSCs.

A Dictyostelium homologue of the metazoan Cbl proteins regulates STAT signalling

Cbl proteins downregulate metazoan signalling pathways by ubiquitylating receptor tyrosine kinases, thereby targeting them for degradation. They contain a phosphotyrosine-binding region, comprising an EF-hand and an SH2 domain, linked to an E3 ubiquitin-ligase domain. CblA, a Dictyostelium homologue of the Cbl proteins, contains all three conserved domains. In a cblA(-) strain early development occurs normally but migrating cblA(-) slugs frequently fragment and the basal disc of the culminants that are formed are absent or much reduced. These are characteristic features of mutants in signalling by DIF-1, the low-molecular-mass prestalk and stalk cell inducer. Tyrosine phosphorylation of STATc is induced by DIF-1 but in the cblA(-) strain this response is attenuated relative to parental cells. We present evidence that CblA fulfils this function, as a positive regulator of STATc tyrosine phosphorylation, by downregulating PTP3, the protein tyrosine phosphatase responsible for dephosphorylating STATc. Thus Cbl proteins have an ancient origin but, whereas metazoan Cbl proteins regulate tyrosine kinases, the Dictyostelium Cbl regulates via a tyrosine phosphatase.

Ligand binding induces Cbl-dependent EphB1 receptor degradation through the lysosomal pathway

Eph receptor tyrosine kinases play a critical role in embryonic patterning and angiogenesis. In the adult, they are involved in carcinogenesis and pathological neovascularization. However, the mechanisms underlying their role in tumor formation and metastasis remain to be defined. Here, we demonstrated that stimulation of EphB1 with ephrinB1/Fc led to a marked downregulation of EphB1 protein, a process blocked by the lysosomal inhibitor bafilomycin. Following ephrinB1 stimulation, the ubiquitin ligase Cbl was recruited by EphB1 and then phosphorylated. Both Cbl phosphorylation and EphB1 ubiquitination were blocked by the Src inhibitor PP2. Overexpression of wild-type Cbl, but not of 70Z mutant lacking ligase activity, enhanced EphB1 ubiquitination and degradation. This negative regulation required the tyrosine kinase activity of EphB1 as kinase-dead EphB1-K652R was resistant to Cbl. Glutathione S-transferase binding experiments showed that Cbl bound to EphB1 through its tyrosine kinase-binding domain. In aggregate, we demonstrated that Cbl induces the ubiquitination and lysosomal degradation of activated EphB1, a process requiring EphB1 and Src kinase activity. To our knowledge, this is the first study dissecting the molecular mechanisms leading to EphB1 downregulation, thus paving the way to new means of modulating their angiogenic and tumorigenic properties.

A role of the protein Cbl in FGF-2-induced angiogenesis in murine brain endothelial cells

The Cbl protein functions both as a multivalent adaptor and a negative regulator of receptor tyrosine kinases (RTKs), the latter by directing polyubiquitination of RTKs. To study the function of Cbl in endothelial cell signalling and angiogenesis, wild-type Cbl and tyrosine kinase binding (TKB) domain mutated Cbl (G306E) were overexpressed in murine immortalised brain endothelial (IBE) cells. Wild-type Cbl cells exhibited enhanced proliferation in low serum compared with the control and G306E Cbl cells. Furthermore, up-regulated phosphorylation of fibroblast growth factor receptor 1 (FGFR-1) and Akt were observed in wild-type Cbl cells upon FGF-2 stimulation. A Cbl TKB domain mutant, G306E, disrupted the phosphorylation of the FGFR-1 but not that of FRS2. In the tubular morphogenesis assay, cells expressing wild-type Cbl initially formed tubular structures. These showed decreased stability and converted into cell aggregates, possibly due to a failure to cease proliferating. Our data support the idea that the wild-type Cbl cells exhibit enhanced proliferation, and thus lose their ability to differentiate appropriately. The present study reveals a role of the Cbl protein in FGF-2 dependent signalling in endothelial cells by its destabilisation of tubular structures.

Interferon-alpha promotes the anti-proliferative effect of gefitinib (ZD 1839) on human colon cancer cell lines

OBJECTIVE

Interferon-alpha (IFN alpha) treatment is associated with up-regulation of epidermal growth factor receptor (HER 1/EGFR) expression and marked growth inhibition of colon cancer cell lines in vitro. We aimed to determine the effect of combining IFN alpha and gefitinib on colon cancer cell line growth.

METHODS

A panel of nine colon cancer cell lines were characterised for expression of HER 1/EGFR and then treated with gefitinib alone, or IFN alpha alone, or IFN alpha plus gefitinib, following a pre-treatment using vehicle or IFN alpha. Crystal violet staining and flow cytometry were used to assess cell proliferation and expression of HER 1/EGFR. The indexes and statistical assays were used to evaluate significant differences between treatment groups against vehicle control.

RESULTS

All cell lines except SW 620 were HER 1/EGFR positive. IFN alpha treatment was associated with significant up-regulation of cell surface HER 1/EGFR expression in all HER 1/EGFR-positive cell lines except KM 12 SM. Concurrent treatment with IFN alpha and gefitinib, or IFN alpha pre-treatment followed by gefitinib, or IFN alpha pre-treatment followed by a combination of IFN alpha plus gefitinib, additively or supra-additively/synergistically enhanced the sensitivity of the seven HER 1/EGFR-up-regulated cell lines.

CONCLUSION

IFN alpha improves the anti-proliferative effect of EGFR inhibition in colorectal cancer cell lines. This approach may have clinical implications for improving treatment based on targeting of HER 1/EGFR.

The SH2 domain: versatile signaling module and pharmaceutical target

The Src homology 2 (SH2) domain is the most prevalent protein binding module that recognizes phosphotyrosine. This approximately 100-amino-acid domain is highly conserved structurally despite being found in a wide variety proteins. Depending on the nature of neighboring protein module(s), such as catalytic domains and other protein binding domains, SH2-containing proteins play many different roles in cellular protein tyrosine kinase (PTK) signaling pathways. Accumulating evidence indicates SH2 domains are highly versatile and exhibit considerable flexibility in how they bind to their ligands. To illustrate this functional versatility, we present three specific examples: the SAP, Cbl and SOCS families of SH2-containing proteins, which play key roles in immune responses, termination of PTK signaling, and cytokine responses. In addition, we highlight current progress in the development of SH2 domain inhibitors designed to antagonize or modulate PTK signaling in human disease. Inhibitors of the Grb2 and Src SH2 domains have been extensively studied, with the aim of targeting the Ras pathway and osteoclastic bone resorption, respectively. Despite formidable difficulties in drug design due to the lability and poor cell permeability of negatively charged phosphorylated SH2 ligands, a variety of structure-based strategies have been used to reduce the size, charge and peptide character of such ligands, leading to the development of high-affinity lead compounds with potent cellular activities. These studies have also led to new insights into molecular recognition by the SH2 domain.

Sex differences in expression of transforming growth factor-α and epidermal growth factor receptor mRNA in waved-1 and C57Bl6 mice

A reduction of transforming growth factor-alpha (TGFalpha) expression in the spontaneous Waved-1 (Wa-1) mutant mouse causes specific behavioral and anatomical changes, including reduced fear learning and stress response and enlarged lateral ventricles. These alterations are observed predominantly in male Wa-1 mice after puberty. We hypothesized that regional differences in the expression of TGFalpha and its receptor, epidermal growth factor receptor (EGFR), may regulate the sexual dimorphism of the brain structures and functions during postnatal development. In general, fear learning-associated structures, including hippocampus and amygdala, showed maximum expression before puberty, regardless of genotype. In contrast, an overall temporal delay in the rise of both transcript levels, which peaked around or after puberty onset, was observed for the major stress regulatory hypothalamo-pituitary-adrenal axis. This pattern of expression was reversed for amygdala EGFR and hypothalamus TGFalpha and EGFR transcripts in males. When regional TGFalpha expression was compared between control and Wa-1 mice, far more complex patterns than expected were observed that revealed sex- and structure-dependent differences. In fact, the amygdala, hypothalamus, and pituitary TGFalpha expression pattern in Wa-1 exhibited a clear sex dependency across various age groups. Surprisingly, there was no compensatory up-regulation of the EGFR transcript in Wa-1 mice. The observed expression patterns of the TGFalpha signaling system during normal development and in the Wa-1 mutant mouse suggest complex sex- and age-dependent transcription regulatory mechanisms.

Changes in epidermal growth factor receptor expression in human bladder cancer cell lines following interferon-alpha treatment

PURPOSE

We examined the regulation of epidermal growth factor (EGF) receptor (EGFR) expression in human bladder cancer cell lines by interferon-alpha (IFN-alpha), the ability of IFN-alpha to inhibit cell proliferation and the sensitivity of IFN-alpha pretreated cells to EGF.

MATERIALS AND METHODS

Cell proliferation was determined using crystal violet colorimetric and clonogenic assays. EGFR expression was measured by flow cytometry using specific antibody or ligand binding approaches.

RESULTS

After IFN-alpha (100 IU/ml) treatment cell surface EGFR expression was upregulated in 6 of 11 and down-regulated in 2 of 11 bladder cancer cell lines. The over expression of cell surface EGFR peaked within 48 to 96 hours and increased by 35% to 241% in individual cell lines. High level cell surface EGFR correlated with intracellular EGFR expression. Cell growth inhibition by IFN-alpha coexisted with EGFR over expression in the 6 lines. IFN-alpha treated cells remained sensitive to EGF treatment.

CONCLUSIONS

IFN-alpha transiently up-regulates EGFR expression and inhibits in vitro growth in some human bladder cancer cells. IFN-alpha does not prevent EGFR from binding EGF or signal transduction via the EGF-EGFR pathway. This may have clinical implications for improving treatment based on EGFR targeting in select patients with bladder cancer.

Role of the JAK/STAT signal transduction pathway in the regulation of gene expression in CNS

Over the last 20 years the JAK/STAT signal transduction pathway has been extensively studied. An enormous amount of data on different cell signal transduction pathways is now available. The JAK/STAT signal transduction pathway is one of the intracellular signaling pathways activated by cytokines and growth factors that was first studied in the hematopoietic system, but recent data demonstrate that this signal transduction is also greatly utilized by other systems. The JAK/STAT pathway is a signaling cascade that links the activation of specific cell membrane receptors to nuclear gene expression. This review is focused on the role of JAK/STAT signal transduction pathway activation in the central nervous system (CNS).

Interplay between integrins and FLK-1 in shear stress-induced signaling

Blood flow can modulate vascular cell functions. We studied interactions between integrins and Flk-1 in transducing the mechanical shear stress due to flow. This application of a step shear stress caused Flk-1. Casitas B-lineage lymphoma (Cbl) activation (Flk-1. Cbl association, tyrosine phosphorylation of the Cbl-bound Flk-1, and tyrosine phosphorylation of Cbl) in bovine aortic endothelial cells (BAECs). The activation of integrins by plating BAECs on vitronectin or fibronectin also induced this Flk-1. Cbl activation. The shear-induced Flk-1. Cbl activation was blocked by inhibitory antibodies for alphavbeta3- or beta1-integrin, suggesting that it is mediated by integrins. Inhibition of Flk-1 by SU1498 also abolished this shear-induced Flk-1. Cbl activation. In contrast to the requirement of integrins for Flk-1. Cbl activation, the Flk-1 blocker SU1498 had no detectable effect on the shear-induced integrin activation, suggesting that integrins and Flk-1 play sequential roles in the signal transduction hierarchy induced by shear stress. Integrins are essential for the mechanical activation of Flk-1 by shear stress but not for the chemical activation of Flk-1 by VEGF.

c-Cbl Is Involved in Met Signaling in B Cells and Mediates Hepatocyte Growth Factor-Induced Receptor Ubiquitination

Hepatocyte growth factor/scatter factor (HGF) and its receptor tyrosine kinase Met are key regulators of epithelial motility and morphogenesis. Recent studies indicate that the HGF/Met pathway also plays a role in B cell differentiation, whereas uncontrolled Met signaling may lead to B cell neoplasia. These observations prompted us to explore HGF/Met signaling in B cells. In this study, we demonstrate that HGF induces strong tyrosine phosphorylation of the proto-oncogene product c-Cbl in B cells and increases Cbl association with the Src family tyrosine kinases Fyn and Lyn, as well as with phosphatidylinositol-3 kinase and CrkL. In addition, we demonstrate that c-Cbl mediates HGF-induced ubiquitination of Met. This requires the juxtamembrane tyrosine Y1001 (Y2) of Met, but not the multifunctional docking site (Y14/15) or any additional C-terminal tyrosine residues (Y13-16). In contrast to wild-type c-Cbl, the transforming mutants v-Cbl and 70Z/3 Cbl, which lack the ubiquitin ligase RING finger domain, suppress Met ubiquitination. Our findings identify c-Cbl as a negative regulator of HGF/Met signaling in B cells, mediating ubiquitination and, consequently, proteosomal degradation of Met, and suggest a role for Cbl in Met-mediated tumorigenesis.

c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription

We have previously demonstrated that cellular stimulation with GH results in the formation of a multiprotein signaling complex. One component of this multiprotein signaling complex is the adapter molecule c-Cbl. Here we have examined the role of c-Cbl in the mechanism of GH signal transduction. Forced expression of c-Cbl in NIH3T3 cells did not alter GH-stimulated Janus kinase 2 tyrosine phosphorylation nor GH-stimulated p44/42 MAPK activation and consequent Elk-1- mediated transcription. c-Cbl overexpression did, however, result in enhanced and prolonged GH-stimulated activation of phosphatidylinositol 3-kinase. Forced expression of c-Cbl did not affect GH-stimulated STAT5 tyrosine phosphorylation, nuclear translocation, nor binding to DNA but markedly abrogated GH-stimulated STAT5-mediated transactivation. c-Cbl overexpression resulted in increased ubiquitination and proteosomal degradation of STAT5 and increased degradation of GH-stimulated tyrosine phosphorylated STAT5. Cellular pretreatment with the proteosomal inhibitor MG132 reversed the effect of c-Cbl overexpression with prolonged duration of GH-stimulated STAT5 tyrosine phosphorylation and restoration of STAT5-mediated transcription. Thus, c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription by direction of STAT5 for proteosomal degradation.

Repression of c-Cbl leads to enhanced G-CSF Jak-STAT signaling without increased cell proliferation

Engagement of the Granulocyte-Colony-Stimulating Factor (G-CSF) receptor activates non-receptor protein tyrosine kinases Lyn and Jak2. We found that Lyn-deficient DT40 cells that express the G-CSF receptor (DT40GR) do not demonstrate G-CSF-induced mitogenic signaling. Lyn associates with and phosphorylates a small set of molecules, including c-Cbl. c-Cbl is an adaptor involved in cell growth and cytoskeletal reorganization, predominantly in hematopoietic cells. Using yeast two-hybrid analysis, we found that c-Cbl directly couples Lyn to PI 3-kinase. We also found that expression of the c-CblY731F mutant, which uncouples PI 3-kinase, resulted in the inhibition of G-CSF-induced proliferative signaling in DT40GR cells. As a complementary strategy, we sought to analyse the effects of c-Cbl deficiency in DT40GR cells. We isolated, cloned and sequenced the full-length cDNA for chicken c-Cbl and constructed antisense vectors. Antisense inhibition of c-Cbl expression in DT40GR cells led to enhanced Jak-STAT activation following G-CSF stimulation. Yet, this enhancement of Jak-STAT activation was associated with decreased G-CSF-induced PI 3-kinase activity and DNA synthesis. PI 3-kinase activity correlated with DNA synthesis and physiological levels of c-Cbl. Together, these data suggest that physiologic level of c-Cbl provides a growth stimulatory pathway for G-CSF and that enhanced Jak-STAT activation is not sufficient for G-CSF-induced growth.

Growth factors and steroid hormones: a complex interplay in the hypothalamic control of reproductive functions

The mechanisms through which LHRH-secreting neurons are controlled still represent a crucial and debated field of research in the neuroendocrine control of reproduction. In the present review, we have specifically considered two potential signals reaching these hypothalamic neurons: steroid hormones and growth factors. Examples of the relevant physiological role of the interactions between these two families of biologically acting molecules have been provided. In many cases, these interactions occur at the level of hypothalamic astrocytes, which are presently accepted as functional partners of the LHRH-secreting neurons. On the basis of the observations here summarized, we have formulated the hypothesis that a functional co-operation of steroid hormones and growth factors occurring in the hypothalamic astrocytic compartment represents a key factor in the neuroendocrine control of reproductive functions.

Regulation of STAT protein synthesis by c-Cbl

Many cytokines and growth factors induce transcription of immediate early response genes by activating members of the Signal Transducers and Activators of Transcription (STAT) family. Although significant progress has been made in understanding the events that lead to the activation of STAT proteins, less is known about the regulation of their expression. Here we report that murine embryonic fibroblasts derived from c-Cbl-deficient mice display significantly increased levels of STAT1 and STAT5 protein. In contrast, STAT2 and STAT3 expression, as well as the levels of the tyrosine kinases Jak1 and Tyk2, appear to be regulated independently of c-Cbl. Interestingly, the half-life of STAT1 was unaffected by the presence of c-Cbl, indicating that c-Cbl acts independently of STAT1 degradation. Further analysis revealed similar levels of STAT1 mRNA, however, a dramatically increased rate of STAT1 protein synthesis was observed in c-Cbl-deficient cells. Thus, our findings demonstrate an additional control mechanism over STAT1 function, and also provide a novel biological effect of the Cbl protein family.

Src-catalyzed phosphorylation of c-Cbl leads to the interdependent ubiquitination of both proteins

The protooncogene c-Cbl has recently emerged as an E3 ubiquitin ligase for activated receptor tyrosine kinases. We report here that c-Cbl also mediates the ubiquitination of another protooncogene, the non-receptor tyrosine kinase c-Src, as well as of itself. The c-Cbl-dependent ubiquitination of Src and c-Cbl requires c-Cbl's RING finger, Src kinase activity, and c-Cbl's tyrosine phosphorylation, probably on Tyr-371. In vitro, c-Cbl forms a stable complex with the ubiquitin-conjugating enzyme UbcH7, but active Src destabilizes this interaction. In contrast, Src inhibition stabilizes the c-Cbl. UbcH7.Src complex. Finally, c-Cbl reduces v-Src protein levels and suppresses v-Src-induced STAT3 activation. Thus, in addition to mediating the ubiquitination of activated receptor tyrosine kinases, c-Cbl also acts as a ubiquitin ligase for the non-receptor tyrosine kinase Src, thereby down-regulating Src.

Suppression of apoptosis and granulocyte colony-stimulating factor-induced differentiation by an oncogenic form of Cbl

OBJECTIVE

The retroviral oncogene v-Cbl causes pre-B cell lymphomas and myeloid leukemias in mice, and its Drosophila homologue is oncogenic, causing enhanced receptor tyrosine kinase signaling. The human Cbl gene resides at 11q23. The aim of this study is to determine the effect of oncogenic Cbl on growth-regulating responses.

MATERIALS AND METHODS

The oncogenic mutant of Cbl (CblDelta1-357) was transfected into factor-dependent 32Dcl3 myeloid cells. Consequently, cell survival and differentiation were measured. Lyn, Syk, MAP kinase, and phosphatidylinositol 3'(PI3')-kinase activities, protein phosphorylation, Bcl-2 promoter activity, ubiquitination, and levels of Bcl-2, Bax, Bad, and Bcl-x(L) were determined. In addition, the effect of v-Cbl on TF-1 cell survival upon granulocyte-macrophage colony-stimulating factor withdrawal was studied.

RESULTS

32Dcl3 and TF-1 cells expressing v-Cbl showed resistance to apoptosis upon growth factor withdrawal, and 32Dcl3 cells completely failed to respond to granulocyte colony-stimulating factor's induction of differentiation. Basal activities of Lyn, Syk, and PI3'-kinase were elevated in the v-Cbl line. There was neither enhanced tyrosine phosphorylation of cellular protein content, Cbl, or Jak2, nor serine phosphorylation of MAP kinase or Akt. After factor withdrawal, the level of Bcl-2 was greater in v-Cbl cells than in control cells.

CONCLUSIONS

Neither increased Bcl-2 promoter activity nor decreased ubiquitination of Bcl-2 could account for increased Bcl-2 levels. v-Cbl-expressing 32Dcl3 cells were resistant to differentiation. v-Cbl suppresses apoptosis and differentiation, possibly through enhancement of Lyn, Syk, and PI3'-kinase activities and Bcl-2.

Cbl: many adaptations to regulate protein tyrosine kinases

Responses to extracellular stimuli are often transduced from cell-surface receptors to protein tyrosine kinases which, when activated, initiate the formation of protein complexes that transmit signals throughout the cell. A prominent component of these complexes is the product of the proto-oncogene c-Cbl, which specifically targets activated protein tyrosine kinases and regulates their signalling. How, then, does this multidomain protein shape the responses generated by these signalling complexes?

L-selectin tyrosine phosphorylates cbl and induces association of tyrosine-phosphorylated cbl with crkl and grb2

L-Selectin-mediated rolling of leukocytes on endothelial cells is an important step for lymphocyte homing and an early event in the immune response to pathogens or inflammatory stimuli. We have previously elucidated intracellular signaling cascades upon L-selectin engagement resulting in activation of Ras, Rac and JNK as well as cytoskeletal changes, oxygen release, ceramide synthesis and receptor capping. Activation of the src-tyrosine kinase p56lck is followed by phosphorylation of the L-selectin molecule and MAP-K. Here we show a tyrosine kinase dependent phosphorylation of the Cbl adapter protein after L-selectin engagement in lymphocytes. Phosphorylation of Cbl was absent in Jurkat cells that are pharmacologically treated with tyrosine kinase inhibitors and in lck-deficient JCaM cells. There is an activation induced association of tyrosine phosphorylated Cbl with Grb2 and CrkL, respectively, but not CrkII. Therefore, the adapter protein Cbl plays a role in L-selectin signaling and might modulate immune function by the specific recruitment of signaling molecules to multiprotein complexes.

What role(s) for TGFalpha in the central nervous system?

Transforming growth factor alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR or erbB1). Identified since 1985 in the central nervous system (CNS), its functions in this organ have started to be determined during the past decade although numerous questions remain unanswered. TGFalpha is widely distributed in the nervous system, both glial and neuronal cells contributing to its synthesis. Although astrocytes appear as its main targets, mediating in part TGFalpha effects on different neuronal populations, results from different studies have raised the possibility for a direct action of this growth factor on neurons. A large array of experimental data have thus pointed to TGFalpha as a multifunctional factor in the CNS. This review is an attempt to present, in a comprehensive manner, the very diverse works performed in vitro and in vivo which have provided evidences for (i) an intervention of TGFalpha in the control of developmental events such as neural progenitors proliferation/cell fate choice, neuronal survival/differentiation, and neuronal control of female puberty onset, (ii) its role as a potent regulator of astroglial metabolism including astrocytic reactivity, (iii) its neuroprotective potential, and (iv) its participation to neuropathological processes as exemplified by astroglial neoplasia. In addition, informations regarding the complex modes of TGFalpha action at the molecular level are provided, and its place within the large EGF family is precised with regard to the potential interactions and substitutions which may take place between TGFalpha and its kindred.

Cbl-transforming variants trigger a cascade of molecular alterations that lead to epithelial mesenchymal conversion

Dispersal of epithelial cells is an important aspect of tumorigenesis, and invasion. Factors such as hepatocyte growth factor induce the breakdown of cell junctions and promote cell spreading and the dispersal of colonies of epithelial cells, providing a model system to investigate the biochemical signals that regulate these events. Multiple signaling proteins are phosphorylated in epithelial cells during hepatocyte growth factor-induced cell dispersal, including c-Cbl, a protooncogene docking protein with ubiquitin ligase activity. We have examined the role of c-Cbl and a transforming variant (70z-Cbl) in epithelial cell dispersal. We show that the expression of 70z-Cbl in Madin-Darby canine kidney epithelial cells resulted in the breakdown of cell-cell contacts and alterations in cell morphology characteristic of epithelial-mesenchymal transition. Structure-function studies revealed that the amino-terminal portion of c-Cbl, which corresponds to the Cbl phosphotyrosine-binding/Src homology domain 2, is sufficient to promote the morphological changes in cell shape. Moreover, a point mutation at Gly-306 abrogates the ability of the Cbl Src homology domain 2 to induce these morphological changes. Our results identify a role for Cbl in the regulation of epithelial-mesenchymal transition, including loss of adherens junctions, cell spreading, and the initiation of cell dispersal.

The proto-oncogene c-Cbl is a positive regulator of Met-induced MAP kinase activation: a role for the adaptor protein Crk

Hepatocyte growth factor triggers a complex biological program leading to invasive cell growth by activating the c-Met receptor tyrosine kinase. Following activation, Met signaling is elicited via its interactions with SH2-containing proteins, or via the phosphorylation of the docking protein Gab1, and the subsequent interaction of Gab1 with additional SH2-containing effector molecules. We have previously shown that the interaction between phosphorylated Gab1 and the adaptor protein Crk mediates activation of the JNK pathway downstream of Met. We report here that c-Cbl, which is a Gab1-like docking protein, also becomes tyrosine-phosphorylated in response to Met activation and serves as a docking molecule for various SH2-containing molecules, including Crk. We further show that Cbl is similarly capable of enhancing Met-induced JNK activation, and several lines of experimentation suggests that it does so by interacting with Crk. We also show that both Cbl and Gab1 enhance Met-induced activation of another MAP kinase cascade, the ERK pathway, in a Crk-independent manner. Taken together, our studies demonstrate a previously unidentified functional role for Cbl in Met signaling and suggest that Met utilizes at least two docking proteins, Gab1 and Cbl, to activate downstream signaling pathways. Oncogene (2000) 19, 4058 - 4065.

Negative regulation of cytokine signaling pathways

The Janus family of protein tyrosine kinases (JAKs) and STAT transcription factors regulate cellular processes involved in cell growth, differentiation, and transformation through their association with cytokine receptors. The CIS family of proteins (also referred to as the SOCS or SSI family) has been implicated in the regulation of signal transduction by a variety of cytokines. Most of them appear to be induced after stimulation with several different cytokines, and at least three of them (CIS1, CIS3/SOCS3, and JAB/SOCS1) negatively regulate cytokine signal transduction by various means: CIS1 inhibits STAT5 activation by binding to cytokine receptors that recruit STAT5, whereas JAB/SOCS-1 and CIS3/SOCS-3 directly bind to the kinase domain of JAKs, thereby inhibiting tyrosine-kinase activity. Therefore, these CIS family members seem to function in a classical negative feedback loop of cytokine signaling. Biochemical characterization as well as gene disruption studies indicate that JAB/SOCS1/SSI-1 is an important negative regulator of interferon gamma signaling. The mechanisms by which these inhibitors of cytokine signal transduction exert their effects have been extensively studied and will provide useful information for regulating tyrosine-kinase activity.

The janus kinase inhibitor, Jab/SOCS-1, is an interferon-gamma inducible gene and determines the sensitivity to interferons

The Janus family of protein tyrosine kinases (JAKs) and STAT transcription factors regulate cellular processes involved in cell growth, differentiation, and transformation through their association with cytokine receptors. The CIS family of proteins (also referred as the SOCS or SSI family) has been implicated in the regulation of signal transduction by a variety of cytokines. Among them, we have shown that JAB/SOCS-1 is strongly induced by interferon-gamma and forced expression of JAB/SOCS-1I conferred cells interferon resistance. This resistance was caused by inhibition of JAK1 and JAK2 activation in response to IFNgamma. Moreover, recent detailed analysis of JAB/SOCS-1 knockout mice revealed that JAB/SOCS-1 is indeed a "negative feedback regulator" that determine the sensitivity of cells to IFNgamma. Using in vitro mutagensis, we defined a functional structure of JAB/SOCS-1 and proposed a mechanism for how JAB inhibits JAK kinase activity.

Differential effects of Cbl and 70Z/3 Cbl on T cell receptor-induced phospholipase Cgamma-1 activity

We demonstrate that the differential effects Cbl and oncogenic 70Z/3 Cbl have on Ca(2+)/Ras-sensitive NF-AT reporters is partially due to their opposing ability to regulate phospholipase Cgamma1 (PLCgamma1) activation as demonstrated by analysis of the activation of an NF-AT reporter construct and PLCgamma1-mediated inositol phospholipid (PI) hydrolysis. Cbl over-expression resulted in reduced T cell receptor-induced PI hydrolysis, in the absence of any effect on PLCgamma1 tyrosine phosphorylation. In contrast, expression of 70Z/3 Cbl led to an increase in basal and OKT3-induced PLCgamma1 phosphorylation and PI hydrolysis. These data indicate that Cbl and 70Z/3 Cbl differentially regulate PLCgamma1 phosphorylation and activation. The implications of these data on the mechanism of Cbl-mediated signaling regulation are discussed.

Association of STATs with relatives and friends

Members of the STAT family of transcription factors are present in species as diverse as mammals, insects and slime molds. Discovered as mediators of interferon-induced signals, the STATs were later shown to drive many different ligand-induced responses through receptor-induced tyrosine phosphorylation and dimerization. STAT1 also functions as a transcription factor, essential for the efficient constitutive expression of certain genes, without needing tyrosine phosphorylation, and phosphorylated STAT1 dimers mediate suppression - rather than activation - of some genes. STATs are present in the cytoplasm of untreated cells in multiprotein complexes, which might aid in their nuclear translocation and differential binding to DNA, thus contributing to the specificity of STAT action. This review explores the diverse protein-protein interactions that underlie the multiple functions of the STATs.

The evolutionarily conserved N-terminal region of Cbl is sufficient to enhance down-regulation of the epidermal growth factor receptor

The mammalian proto-oncoprotein Cbl and its homologues in Caenorhabditis elegans and Drosophila are evolutionarily conserved negative regulators of the epidermal growth factor receptor (EGF-R). Overexpression of wild-type Cbl enhances down-regulation of activated EGF-R from the cell surface. We report that the Cbl tyrosine kinase-binding (TKB) domain is essential for this activity. Whereas wild-type Cbl enhanced ligand-dependent EGF-R ubiquitination, down-regulation from the cell surface, accumulation in intracellular vesicles, and degradation, a Cbl TKB domain-inactivated mutant (G306E) did not. Furthermore, the transforming truncation mutant Cbl-N (residues 1-357), comprising only the Cbl TKB domain, functioned as a dominant negative protein. It colocalized with EGF-R in intracellular vesicular structures, yet it suppressed down-regulation of EGF-R from the surface of cells expressing endogenous wild-type Cbl. Therefore, Cbl-mediated down-regulation of EGF-R requires the integrity of both the N-terminal TKB domain and additional C-terminal sequences. A Cbl truncation mutant comprising amino acids 1-440 functioned like wild-type Cbl in down-regulation assays. This mutant includes the evolutionarily conserved TKB and RING finger domains but lacks the less conserved C-terminal sequences. We conclude that the evolutionarily conserved N terminus of Cbl is sufficient to effect enhancement of EGF-R ubiquitination and down-regulation from the cell surface.

Ligand-induced ubiquitination of the epidermal growth factor receptor involves the interaction of the c-Cbl RING finger and UbcH7

c-Cbl plays a negative regulatory role in tyrosine kinase signaling by an as yet undefined mechanism. We demonstrate here, using the yeast two-hybrid system and an in vitro binding assay, that the c-Cbl RING finger domain interacts with UbcH7, a ubiquitin-conjugating enzyme (E2). UbcH7 interacted with the wild-type c-Cbl RING finger domain but not with a RING finger domain that lacks the amino acids that are deleted in 70Z-Cbl, an oncogenic mutant of c-Cbl. The in vitro interaction was enhanced by sequences on both the N- and C-terminal sides of the RING finger. In vivo and in vitro experiments revealed that c-Cbl and UbcH7 synergistically promote the ligand-induced ubiquitination of the epidermal growth factor receptor (EGFR). In contrast, 70Z-Cbl markedly reduced the ligand-induced, UbcH7-mediated ubiquitination of the EGFR. MG132, a proteasome inhibitor, significantly prolonged the ligand-induced phosphorylation of both the EGFR and c-Cbl. Thus, c-Cbl plays an essential role in the ligand-induced ubiquitination of the EGFR by a mechanism that involves an interaction of the RING finger domain with UbcH7. This mechanism participates in the down-regulation of tyrosine kinase receptors and loss of this function, as occurs in the naturally occurring 70Z-Cbl isoform, probably contributes to oncogenic transformation.

Molecular cloning and characterization of a novel cbl-family gene, cbl-c

We have cloned a novel gene, cbl-c, of mammalian cbl-family. The cbl-c gene is predicted to encode a protein of 52 kDa that has a phosphotyrosine-binding domain, a RING finger and a proline-rich region. Cbl-c shows 50% homology to the amino-terminal sequences of Cbl and Cbl-b, but a sequence corresponding to the carboxy-terminal half of Cbl and Cbl-b is largely missing in Cbl-c. The expression of cbl-c mRNA is distinct from that of cbl and cbl-b mRNAs, being high in the colon and small intestine, but undetectable in brain and lymphoid tissues. The cbl-c gene is mapped in 19q13.2-13.3. Finally, the 52 kDa Cbl-c protein binds to the EGF receptor and Fyn tyrosine kinase. We conclude that Cbl-c is a novel Cbl-family adaptor protein that would regulate intracellular signaling mediated by various tyrosine kinases.

The oncogenic 70Z Cbl mutation blocks the phosphotyrosine binding domain-dependent negative regulation of ZAP-70 by c-Cbl in Jurkat T cells

T-cell receptor (TCR) engagement results in the activation of Src family (Lck and Fyn) and ZAP-70 protein tyrosine kinases, leading to tyrosine phosphorylation of multiple cellular substrates including the complex adapter protein c-Cbl. Moreover, Cbl is tyrosine phosphorylated upon engagement of growth factor receptors, cytokine receptors, and immunoreceptors and functions as a negative regulator of tyrosine kinase signalling pathways. Cbl associates via its phosphotyrosine binding (PTB) domain to the ZAP-70 pY292 negative regulatory phosphotyrosine. We recently demonstrated that the oncogenic Cbl mutant, 70Z Cbl, requires its PTB domain to upregulate NFAT in unstimulated Jurkat T cells. Here, we demonstrate that kinase-dead but not wild-type forms of Fyn, Lck, and ZAP-70 block 70Z Cbl-mediated NFAT activation. Moreover, 70Z Cbl does not upregulate NFAT in the ZAP-70-deficient P116 Jurkat T-cell line. The requirement for Fyn, Lck, and ZAP-70 is not due to tyrosine phosphorylation of 70Z Cbl, as mutation of all tyrosines in, or deletion of, the C-terminal region of 70Z Cbl (amino acids 655 to 906) blocks 70Z Cbl tyrosine phosphorylation but enhances 70Z Cbl-mediated NFAT activation. Further, 70Z Cbl does not cooperate with ZAP-70 Y292F to upregulate NFAT, indicating that 70Z Cbl and ZAP-70 do not activate parallel signalling pathways. Finally, the upregulation of NFAT observed upon ZAP-70 overexpression is blocked by Cbl in a PTB domain-dependent manner. We conclude that oncogenic 70Z Cbl acts as a dominant negative to block the PTB domain-dependent negative regulatory role of endogenous Cbl on ZAP-70, leading to constitutive ZAP-70 signalling and activation of transcription factors.

Role of protein kinase C in signal attenuation following T cell receptor engagement

T lymphocyte activation through stimulation of the T cell receptor complex and co-stimulatory receptors is associated with acute tyrosine phosphorylation of intracellular proteins, which in turn mediate downstream signaling events that regulate interleukin-2 expression and cell proliferation. The extent of protein tyrosine phosphorylation is rapidly attenuated after only 1-2 min of stimulation as a means of tightly controlling the initial signaling response. Here we show that this attenuation of tyrosine phosphorylation of Shc, CrkL, and the proto-oncogene Cbl is mimicked by treatment of mouse T lymphocytes or cultured Jurkat cells with phorbol 12-myristate 13-acetate. This effect is blocked by the specific protein kinase C inhibitor GF109203X, but not by PD98059, an inhibitor of MEK1/2 kinase. Activation of protein kinase C by phorbol ester also causes rapid (t(1)/(2) = 2 min) dissociation of both CrkL and p85/phosphoinositide 3-kinase from Cbl concomitant with Cbl tyrosine dephosphorylation. More important, GF109203X treatment of Jurkat cells prior to T cell receptor stimulation by anti-CD3/CD4 antibodies results in an enhanced (2-fold) peak of Cbl phosphorylation compared with that observed in control cells. Furthermore, the rate of attenuation of both Cbl tyrosine phosphorylation and its association with CrkL following stimulation with anti-CD3/CD4 antibodies is much slower in Jurkat cells treated with GF109203X. Taken together, these data provide strong evidence that one or more isoforms of phorbol ester-responsive protein kinase C play a key role in a feedback mechanism that attenuates tyrosine phosphorylation of proteins and reverses formation of signaling complexes in response to T cell receptor activation.

Protein Kinase C Activation Inhibits Tyrosine Phosphorylation of Cbl and Its Recruitment of Src Homology 2 Domain-Containing Proteins

One of the major proteins that is rapidly tyrosine phosphorylated upon stimulation of the TCR/CD3 complex is the 120-kDa product of the c-cbl protooncogene (Cbl). Upon activation, tyrosine-phosphorylated Cbl interacts with the Src homology 2 (SH2) domains of several signaling proteins, e.g., phosphatidylinositol 3-kinase (PI3-K) and CrkL. In the present study, we report that pretreatment of Jurkat T cells with PMA reduced the anti-CD3-induced tyrosine phosphorylation of Cbl and, consequently, its activation-dependent association with PI3-K and CrkL. A specific protein kinase C (PKC) inhibitor (GF-109203X) reversed the effect of PMA on tyrosine phosphorylation of Cbl and restored the activation-dependent association of Cbl with PI3-K and CrkL. We also provide evidence that PKCα and PKCθ can physically associate with Cbl and are able to phosphorylate it in vitro and in vivo. Furthermore, a serine-rich motif at the C terminus of Cbl, which is critical for PMA-induced 14-3-3 binding, is also phosphorylated by PKCα and PKCθ in vitro. These results suggest that, by regulating tyrosine and serine phosphorylation of Cbl, PKC is able to control the association of Cbl with signaling intermediates, such as SH2 domain-containing proteins and 14-3-3 proteins, which may consequently result in the modulation of its function.

Perturbed Regulation of ZAP-70 and Sustained Tyrosine Phosphorylation of LAT and SLP-76 in c-Cbl-Deficient Thymocytes

Recent studies indicate that c-Cbl and its oncogenic variants can modulate the activity of protein tyrosine kinases. This finding is supported by studies showing that c-Cbl interacts directly with a negative regulatory tyrosine in ZAP-70, and that the levels of tyrosine-phosphorylated ZAP-70 and numerous other proteins are increased in TCR-stimulated thymocytes from c-Cbl-deficient mice. Here, we demonstrate that this enhanced phosphorylation of ZAP-70 and that of two substrates, LAT and SLP-76, is not due to altered protein levels but is the consequence of two separate events. First, we find increased expression of tyrosine-phosphorylated TCRζ chain in c-Cbl-deficient thymocytes, which results in a higher level of ζ-chain-associated ZAP-70 that is initially accessible for activation. Thus, more ZAP-70 is activated and more of its substrates (LAT and SLP-76) become tyrosine-phosphorylated after TCR stimulation. However, an additional mechanism of ZAP-70 regulation is evident at a later time poststimulation. At this time, ZAP-70 from both normal and c-Cbl−/− thymocytes becomes hyperphosphorylated; however, only in normal thymocytes does this correlate with ZAP-70 down-regulation and a diminished ability to phosphorylate LAT and SLP-76. In contrast, c-Cbl-deficient thymocytes display altered phosphorylation kinetics, for which LAT phosphorylation is increased and SLP-76 phosphorylation is sustained. Thus, the ability to down-regulate the phosphorylation of two ZAP-70 substrates is impaired in c-Cbl−/− thymocytes. These findings provide evidence that c-Cbl is involved in the negative regulation of the phosphorylation of LAT and SLP-76 by ZAP-70.

Cbl-mediated negative regulation of platelet-derived growth factor receptor-dependent cell proliferation. A critical role for Cbl tyrosine kinase-binding domain

The Cbl proto-oncogene product has emerged as a novel negative regulator of receptor and non-receptor tyrosine kinases. Our previous observations that Cbl overexpression in NIH3T3 cells enhanced the ubiquitination and degradation of the platelet-derived growth factor receptor-alpha (PDGFRalpha) and that the expression of oncogenic Cbl mutants up-regulated the PDGFRalpha signaling machinery strongly suggested that Cbl negatively regulates PDGFRalpha signaling. Here, we show that, similar to PDGFRalpha, selective stimulation of PDGFRbeta induces Cbl phosphorylation, and its physical association with the receptor. Overexpression of wild type Cbl in NIH3T3 cells led to an enhancement of the ligand-dependent ubiquitination and subsequent degradation of the PDGFRbeta, as observed with PDGFRalpha. We show that Cbl-dependent negative regulation of PDGFRalpha and beta results in a reduction of PDGF-induced cell proliferation and protection against apoptosis. A point mutation (G306E) that inactivates the tyrosine kinase binding domain in the N-terminal transforming region of Cbl compromised the PDGF-inducible tyrosine phosphorylation of Cbl although this mutant could still associate with the PDGFR. More importantly, the G306E mutation abrogated the ability of Cbl to enhance the ligand-induced ubiquitination and degradation of the PDGFR and to inhibit the PDGF-dependent cell proliferation and protection from apoptosis. These results demonstrate that Cbl can negatively regulate PDGFR-dependent biological responses and that this function requires the conserved tyrosine kinase binding domain of Cbl.

cbl-3: a new mammalian cbl family protein

We have cloned a new human gene, cbl-3, which encodes a protein with marked homology to the cbl family of proteins. The predicted protein encoded by this gene retains the conserved phosphotyrosine binding domain (PTB) in the N-terminal and the zinc finger but is significantly shorter (MW 52.5 kDa) than the other mammalian cbl proteins. The protein lacks the extensive proline rich domain and leucine zipper seen in c-cbl and cbl-b and structurally most resembles the C. elegans and Drosophila cbl proteins. The gene is ubiquitously expressed with highest expression in the aerodigestive tract, prostate, adrenal gland, and salivary gland. The protein is phosphorylated and recruited to the EGFR upon EGF stimulation and inhibits EGF stimulated MAP kinase activation. In comparison to the other mammalian cbl proteins (e.g. cbl-b), cbl-3 interacts with a restricted range of proteins containing Src Homology 3 regions. An alternatively spliced form of the cbl-3 protein was also identified which deletes a critical region of the PTB domain and which does not interact with the EGFR nor inhibit EGF stimulated MAP kinase activation. These data demonstrate that cbl-3, a novel mammalian cbl protein, is a regulator of EGFR mediated signal transduction.

Modulation of Integrin Function in Hematopoietic Progenitor Cells by CD43 Engagement: Possible Involvement of Protein Tyrosine Kinase and Phospholipase C-γ

Attachment of cells to extracellular matrix components is critical for the regulation of hematopoiesis. CD43 is a mucin-like transmembrane sialoglycoprotein expressed on the surface of almost all hematopoietic cells. A highly extended structure of extracellular mucin with negative charge may function as a repulsive barrier to hematopoietic cells. However, some investigators have shown that CD43 has proadhesive properties, and engagement of CD43 has been reported to upregulate integrin-mediated cell adhesion in T cells. We found that cross-linking of CD43 with monoclonal antibodies (MoAbs) enhanced integrin 4β1 (very late antigen [VLA]-4) and 5 β1 (VLA-5)-dependent adhesion of human cord blood CD34+ cells to fibronectin. CD34+ CD38hi, but not CD34+CD38−/low cells responded significantly to the stimulus, suggesting that committed, but not stem and more immature progenitors are sensitive to CD43-mediated activation of integrin. To elucidate the molecular mechanism leading to integrin activation, we used the growth factor-dependent cell line MO7e. Cross-linking of CD43 induced tyrosine phosphorylation of several intracellular molecules including the protein tyrosine kinase Syk, the proto-oncogene product Cbl, and phospholipase C (PLC)-γ2 in MO7e cells. Moreover, protein tyrosine kinase inhibitor herbimycin A and PLC inhibitor U73122 both blocked CD43-induced enhancement of adhesion to fibronectin. These results indicate that signals mediated through CD43 may increase integrin affinity to fibronectin via a pathway dependent on protein tyrosine kinase and PLC-γ activation in hematopoietic progenitors.

The proto-oncogene c-Cbl is a negative regulator of DNA synthesis initiated by both receptor and cytoplasmic tyrosine kinases

In C. elegans, genetic and biochemical data indicate that the Cbl homolog Sli-1 attenuates Let-23 (EGFR) signaling. To investigate whether c-Cbhl might have a role in mammalian growth factor-mediated mitogenic signaling, we microinjected NIH3T3 mouse fibroblasts with expression plasmids encoding wt and G306ECbl (a 'loss of function' mutant identified in C. elegans). We observed inhibition of PDGF BB- and EGF-induced DNA synthesis by wt Cbl but not the mutant. Microinjection of two different affinity purified polyclonal antisera against Cbl boosted a suboptimal PDGF-stimulated mitogenic response. The inhibition of both PDGF BB- and EGF-induced DNA synthesis by wt Cbl was reversed by co-expression with Myc but not with Fos. DNA synthesis initiated by constitutively activated Src was also blocked by Cbl expression, but curiously by the G306E mutant as well. These data are all consistent with the proposition that Cbl negatively affects mitogenic signaling in mammalian fibroblasts.

cbl-b inhibits epidermal growth factor receptor signaling

The role of cbl-b in signaling by the epidermal growth factor receptor (EGFR) was studied and compared with c-cbl. We demonstrate in vivo, that cbl-b, like c-cbl, is phosphorylated and recruited to the EGFR upon EGF stimulation and both cbl proteins can bind to the Grb2 adaptor protein. To investigate the functional role of cbl proteins in EGFR signaling, we transfected cbl-b or c-cbl into 32D cells overexpressing the EGFR (32D/EGFR). This cell line is absolutely dependent on exogenous IL-3 or EGF for sustained growth. 32D/EGFR cells overexpressing cbl-b showed markedly inhibited growth in EGF compared to c-cbl transfectants and vector controls. This growth inhibition by cbl-b was the result of a dramatic increase in the number of cells undergoing apoptosis. Consistent with this finding, cbl-b overexpression markedly decreased the amplitude and duration of AKT activation upon EGF stimulation compared to either vector controls or c-cbl overexpressing cells. In addition, the duration of EGF mediated MAP kinase and Jun kinase activation in cells overexpressing cbl-b is shortened. These data demonstrate that cbl-b inhibits EGF-induced cell growth and that cbl-b and c-cbl have distinct roles in EGF mediated signaling.

A role for transforming growth factor alpha as an inducer of astrogliosis

TGFalpha is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR). Synthesis of TGFalpha and EGFR in reactive astrocytes developing after CNS insults is associated with the differentiative and mitogenic effects of TGFalpha on cultured astrocytes. This suggests a role for TGFalpha in the development of astrogliosis. We evaluated this hypothesis using transgenic mice bearing the human TGFalpha cDNA under the control of the zinc-inducible metallothionein promoter. Expression levels of glial fibrillary acidic protein (GFAP) and vimentin and morphological features of astrocytes were used as indices of astroglial reactivity in adult transgenic versus wild-type mice provided with ZnCl2 in their water for 3 weeks. In the striatum, the hippocampus, and the cervical spinal cord, the three CNS areas monitored, transgenic mice displayed enhanced GFAP mRNA and protein levels and elevated vimentin protein levels. GFAP-immunoreactive astrocytes exhibited numerous thick processes and hypertrophied somata, which are characteristic aspects of reactive astrocytes. Their number increased additionally in the striatum and the spinal cord, but no astrocytic proliferation was observed using bromodeoxyuridine immunohistochemistry. Neither the morphology nor the number of microglial cells appeared modified. A twofold increase in phosphorylated EGFR was detected in the striatum and was associated with the immunohistochemical detection of numerous GFAP-positive astrocytes bearing the EGFR, suggesting a direct action of TGFalpha on astrocytes. Altogether, these results demonstrate that enhanced TGFalpha synthesis is sufficient to trigger astrogliosis throughout the CNS, whereas microglial metabolism is unaffected.

APS, an adaptor protein containing PH and SH2 domains, is associated with the PDGF receptor and c-Cbl and inhibits PDGF-induced mitogenesis

Previously we cloned a novel adaptor protein, APS (adaptor molecules containing PH and SH2 domains) which was tyrosine phosphorylated in response to c-kit or B cell receptor stimulation. Here we report that APS was expressed in some human osteosarcoma cell lines, markedly so in SaOS-2 cells, and was tyrosine-phosphorylated in response to several growth factors, including platelet derived growth factor (PDGF), insulin-like growth factor (IGF), and granulocyte-macrophage colony stimulating factor (GM-CSF). Ectopic expression of the wild type APS, but not C-terminal truncated APS, in NIH3T3 fibroblasts suppressed PDGF-induced MAP kinase (Erk2) activation, c-fos and c-myc induction as well as cell proliferation. In vitro binding experiments suggest that APS bound to the beta type PDGF receptor, mainly via phosphotyrosine 1021 (pY1021). Indeed, tyrosine phosphorylation of PLC-gamma, which has been demonstrated to bind to pY1021, but not that of PI3 kinase and associated proteins, was reduced in APS transformants. PDGF induced phosphorylation of the tyrosine residue of APS close to the C-terminal end. In vitro and in vivo binding experiments indicate that the tyrosine phosphorylated C-terminal region of APS bound to c-Cbl, which has been shown to be a negative regulator of tyrosine kinases. Since coexpression of c-Cbl with wild type APS, but not C-terminal truncated APS, synergistically inhibited PDGF-induced c-fos promoter activation, c-Cbl could be a mechanism of inhibitory action of APS on PDGF receptor signaling.

Cbl-mediated negative regulation of the Syk tyrosine kinase. A critical role for Cbl phosphotyrosine-binding domain binding to Syk phosphotyrosine 323

The proto-oncogene product Cbl has emerged as a potential negative regulator of the Syk tyrosine kinase; however, the nature of physical interactions between Cbl and Syk that are critical for this negative regulation remains unclear. Here we show that the phosphotyrosine-binding (PTB) domain within the N-terminal transforming region of Cbl (Cbl-N) binds to phosphorylated Tyr323 in the linker region between the Src homology 2 and kinase domains of Syk, confirming recent results by another laboratory using the yeast two-hybrid approach (Deckert, M., Elly, C., Altman, A., and Liu, Y. C. (1998) J. Biol. Chem. 273, 8867-8874). A PTB domain-inactivating point mutation (G306E), corresponding to a loss-of-function mutation in the Caenorhabditis elegans Cbl homologue SLI-1, severely compromised Cbl-N/Syk binding in vitro and Cbl/Syk association in transfected COS-7 cells. Using heterologous expression in COS-7 cells, we investigated the role of Cbl PTB domain binding to Syk Tyr323 in the negative regulation of Syk. Co-expression of Cbl with Syk in COS-7 cells led to a dose-dependent decrease in the autophosphorylated pool of Syk and in phosphorylation of an in vivo substrate, CD8-zeta. Unexpectedly, these effects were largely due to the loss of Syk protein. Both the decrease in Syk and CD8-zeta phosphorylation and reduction in Syk protein levels were blocked by either G306E mutation in Cbl or by Y323F mutation in Syk. These results demonstrate a critical role for the Cbl PTB domain in the recruitment of Cbl to Syk and in Cbl-mediated negative regulation of Syk.